1. Field of the Invention
The present invention relates generally to semiconductor wafer cleaning. More specifically, the present invention relates to a method and apparatus for decoupling cavitation from power used to induce the cavitation.
2. Description of the Related Art
In the manufacture of semiconductor devices, a surface of a semiconductor wafer (“wafer”) must be cleaned to remove particulate contamination. If particulate contamination is not removed, semiconductor devices on the wafer may perform poorly or become defective. Particulate contamination generally consists of tiny bits of distinctly defined material having an affinity to adhere to the surface of the wafer. Examples of particulate contamination can include silicon dust, silica, slurry residue, polymeric residue, metal flakes, atmospheric dust, plastic particles, and silicate particles, among others.
Megasonic cleaning is one process for removing particulate contamination from the surface of the wafer. During megasonic cleaning, the wafer is placed in a liquid that is subjected to an acoustic energy. In general, the acoustic energy provides two mechanisms to enhance the removal of the particulate contamination, a cavitation mechanism and an acoustic pressure mechanism. Both the cavitation and the acoustic pressure mechanisms occur along a path traversed by sonic waves generated by the acoustic energy.
Cavitation occurs when the acoustic energy applied to the liquid causes a dissolved gas in the liquid to come out of solution to form gas bubbles. During cavitation, the gas bubbles continuously form and collapse. An amount and a size of the gas bubbles generally depends on an amount of the acoustic energy applied to the liquid, the frequency of the acoustic energy, the liquid type, and the amount of dissolved gas in the liquid. As the gas bubbles collapse on or near the wafer surface, energy from the collapsing gas bubbles can be transferred to the wafer surface, including to the particulate contamination adhering to the wafer surface. The transferred energy from the collapsing gas bubbles can be effective at loosening and removing the particulate contamination.
Acoustic streaming resulting from application of the acoustic energy can cause a macroscopic transport (i.e., mass transport) of the liquid to occur at an interface between the surface of the wafer and the liquid. The macroscopic transport of the liquid causes the surface of the wafer to be more rapidly exposed to fresh chemical reactants within the liquid. Also, the macroscopic transport of the liquid causes by-products of reactions to be removed from the surface of the wafer. In this manner, the acoustic streaming can enhance a chemical reaction rate between the liquid and the surface of the wafer. Thus, the acoustic streaming can be beneficial for cleaning processes that utilize chemical reactions to assist in removal of the particulate contamination.
FIG. 1 is an illustration showing a wafer cleaning apparatus, in accordance with the prior art. The wafer cleaning apparatus includes a tank 101 defined by walls and a bottom. During operation, the tank 101 is filled with a liquid 103 exposed to atmospheric pressure. The liquid 103 is provided from a liquid supply and enters the tank bottom as indicated by an arrow 105. A valve 107 is used to control a flow of the liquid 103 into the tank 101. The liquid 103 can also be removed from the tank 101 through a liquid drain as indicated by an arrow 109. A valve 111 is used to control a flow of the liquid 103 from the tank 101.
The tank 101 is further configured with a piezoelectric transducer 115. During operation, the piezoelectric transducer 115 introduces an acoustic energy 117 into the liquid 103 in the tank 101. The acoustic energy 117 causes a plurality of cavitation bubbles 119 to be created within the liquid 103. The plurality of cavitation bubbles 119 are created throughout the liquid 103 within the tank 101. Some of the plurality of cavitation bubbles 119 may approach or contact a wafer 121 submerged within the liquid 103, or be formed on a surface of the wafer 121. Collapsing of the plurality of cavitation bubbles 119 near or on the surface of the wafer 121 will produce energy capable of causing particulate contamination on the wafer 121 to be dislodged and removed. Although the acoustic energy 117 can be beneficial in removing the particulate contamination, the acoustic energy 117 can also cause damage to the wafer 121.
FIG. 2 is an illustration showing several types of wafer damage that can be caused by excessive or uncontrolled cavitation, or excessive or uncontrolled acoustic pressure, in accordance with the prior art. When acoustic energy is applied to a liquid through a transducer, both acoustic streaming and cavitation occur. When a cavitation bubble collapses, a liquid in an immediate vicinity of the cavitation bubble is rapidly accelerated resulting in creation of a pressure wave. A collision between the pressure wave and the wafer can result in damage to structures present on the wafer surface. Furthermore, in addition to increasing cavitation, increasing the acoustic energy also increases acoustic streaming and acoustic pressure. The acoustic pressure can also create significant damage to structures and is believed to be an important damage-causing mechanism. In FIG. 2, a wafer 201 is shown having a structure 203 that has been subjected to delamination damage. The delamination damage can be caused by a pressure wave created by a cavitation bubble collapse 205 on or near the structure 203, or by acoustic pressure from acoustic streaming. Also in FIG. 2, a wafer 211 is shown having a structure 213 that has been subjected to erosion damage. The erosion damage can be caused by a pressure wave created by a cavitation bubble collapse 215 on or near the structure 213, or by acoustic pressure from acoustic streaming. FIG. 2 also shows a wafer 221 having a structure 223 that has been subjected to distortion damage. The distortion damage can be caused by a pressure wave created by a cavitation bubble collapse 225 on or near the structure 223, or by acoustic pressure from acoustic streaming.
In some wafer cleaning processes, it may be desirable to rely more on the acoustic pressure rather than the cavitation to enhance removal of the particulate contamination. These wafer cleaning processes may require the use of a greater acoustic energy to provide the necessary acoustic pressure. However, use of the greater acoustic energy may create cavitation that is capable of damaging the wafer. One known method for limiting the amount of cavitation involves extracting dissolved gas from the liquid to be used in the cleaning process. This method inevitably requires the use of more expensive liquid materials that may not be readily available. Therefore, a need exists to control the cavitation independently from acoustic energy and liquid chemistry.
In other wafer cleaning processes, it may be desirable to rely more on the cavitation rather than the acoustic pressure to enhance removal of the particulate contamination. These wafer cleaning processes may also require the acoustic pressure to be limited due to a fragility of the wafer. Thus, the acoustic energy applied to the liquid must be limited. However, a greater acoustic energy may be required to provide the necessary cavitation.
In view of the foregoing, there is a need for an apparatus and method that can be implemented to decouple and independently control cavitation from acoustic energy in megasonic cleaning applications.